The use of multiple channel cochlear prostheses to improve the communication abilities of profoundly deaf patients is now an established procedure. The central problems of this technique are the provision of a number of perceptually independent information channels and the provision of reliable and usable speech information with these channels. This application aims to provide a versatile and precisely controlled implantable multiple-channel stimulator using custom designed silicon chip technology. The stimulator will simultaneously control the electric currents between three pairs of electrodes chosen from a linear array of 22 electrodes. A voltage monitoring circuit will be switchable to any of the 22 electrodes or to crucial nodes of the circuit to provide in vivo verification of the performance of the device and direct measurement of electrode impedances. The stimulator will be implanted in at least three patients. A psychophysical study will investigate the interactions between simultaneously stimulated groups of electrodes in different positions along the cochlea. The patients' perceptions of spatially complex current distributions will be investigated. A programmable, portable speech processor will be developed concurrently to evaluate the presentation of the frequencies and amplitudes of three peaks derived from the speech signal. The frequencies will be coded as the positions of three simultaneously stimulated electrode pairs in the cochlea. The stimulation waveform will be a biphasic pulse, with the pulse rate derived from the fundamental frequency of the speaker's voice. Speech studies with the patients will compare the performance of this speech processing strategy with the presently used strategy that uses only a single spectral peak. The programmable speech processor will allow optimisation of performance for different patients as well as easy conversion from one speech coding strategy to another. The complete cochlear prosthesis system will provide a powerful tool for the evaluation of further alternative strategies with the aim of providing good speech comprehension for profoundly deaf patients without the use of lipreading.